KPOLAR — Use Kurtz’s method for computing nonlinear optical properties in the inertial frame.

KPOLAR=`n.n`

This keyword activates the polarizability (non-linear optical properties) calculation
method contributed by Henry Kurtz. See
Chapter 12, *Polarizability Methods*
for a description of this method and an example. (See also
APOLAR). The value n.n
is the finite field strength and should only be adjusted by experienced users. The
default value for n.n is 0.004 au (0.20568 V/cm)
and 0.1 au < n.n < 1 × 10^{-5} au.

APOLAR, KPOLAR, and BRUTEKPOLAR all compute polarizability and hyperpolarizability. Both KPOLAR and BRUTEKPOLAR use the finite difference method of Henry Kurtz to compute these properties. APOLAR instead relies on the analytical gradient of the energy with respect to the an electric field and so is more reliable and less sensitive to the finite field strength. The only difference between KPOLAR and BRUTEKPOLAR is that KPOLAR reports its results in the inertial frame while BRUTEKPOLAR reports its results in the genuine Cartesian frame. APOLAR gives its results in both the inertial frame and the genuine Cartesian frame (at no extra cost). APOLAR is faster than KPOLAR and BRUTEKPOLAR because it relies on analytic gradients.

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